The principle of vaccination is essentially based on two key elements of immunity, namely specificity and memory. Activation and differentiation of B cells in response to most antigens requires various signals that drive B cells to form either antibody secreting plasma cells or memory B cells poised to mediate a more rapid response upon secondary exposure to antigen. Memory cells allow the immune system to mount a much stronger response on the second encounter with antigens. This secondary response is both faster to appear and more effective than the primary response. However, because antibodies by nature are very specific, and in view of the diversity of infectious agents, it has still remained a significant problem to develop antibodies that exhibit cross-reactivity across or within the numerous different types of pathogens.
One example of an infectious agent for which there remains a significant challenge to develop antibodies that exhibit cross-reactivity is Haemophilus parasuis (H. parasuis), the etiological agent of porcine polyserositis and arthritis (Glasser's disease). H. parasuis is a Gram-negative, occasionally-capsulated, non-motile, pleomorphic bacterium isolated from serous exudates of swine affected by serofibrinous pleuritis, pericarditis, peritonitis, arthritis, and meningitis. This organism, which was initially described by Glässer in 1910, was likely isolated for the first time by Schermer and Ehrlich in 1922, though the suspect organism was originally referred to as Haemophilus suis. In 1969, however, Biberstein and White showed that the causative agent of Glässers required only nicotinamide adenine dinucleotide (NAD). Haemophilus suis, an organism requiring both iron porphyrin and nicotinamide adenine dinucleotide (NAD), was therefore not the insidious character in this disease and the new organism was renamed, by the addition of the prefix “para”, to H. parasuis. 
H. parasuis characterization has evolved significantly over the past five decades. Bakos, et al. used a precipitation test to identify four serovars, which he designated A-D (Nordic Veterinary Medicine, 4:241-255 (1952)). These four grew to seven in 1986 (J Clin Microbiol, 23:1022-1025 (1986)). Kielstein et al., Zentralbl Veterinarmed B, 38:315-320 (1991) added six more and, working with Rapp-Gabrielson, Am J Vet Res, 53:659-664 (1992), yet another five. Eventually, this classification was refined. All of the serovars, including the few with multiple designations, were characterized based on an immunodiffusion test performed with specific rabbit sera. The result was a list of at least fifteen serovars that have become accepted globally. Unfortunately, a significant number of untypeable isolates also exist. Furthermore, a number of publications have described the serotype profiles of H. parasuis in specific countries. It has been proposed that in the USA, Germany, Japan, Spain, Canada, and China, serotypes 4 and 5 are quite common. Serotypes 5 and 13 have been reported to be prevalent in Australia and Denmark.
The virulence factors of H. parasuis have not been defined. Most associations to virulence are made according to serotype, as some correspond to higher morbidity and mortality rates. Upon intraperitoneal infection, serotypes 1, 5, 10, 12, 13, and 14 have been reported to cause high morbidity and morality rates within 4 day. As such, these strains are considered highly virulent. Three serotypes (i.e., 2, 4, and 15) presented intermediate levels of virulence by causing polyserositis without mortality. The remaining serotypes are considered avirulent as affected swine did not manifest clinical disease.
Attempts have been made to determine specific virulence factors of H. parasuis. Being a member of the Pasteurellaceae family, it was thought that some candidates would include capsules, fimbriae, lipopolysaccharides (LPS), and outer membrane proteins (OMPs). At present, however, few correlations can be drawn between these traits in H. parasuis and virulence. Encapsulated strains are common in both the nasal cavities of healthy swine and clinically manifesting animals. LPS's importance was somewhat debunked by reports suggesting no significant difference in LPS production between virulent and avirulent serotypes and showing that presentations containing both LPS and OMPs elicited responses to the OMPs exclusively.
OMPs have been shown to generate a strong humoral response and candidates for protective immunogens from this category have been proposed. Two general profiles of OMPs are present and may be associated with virulence. Most virulent serotypes are characterized by the second profile, which is dominated by a 37 kDa protein. Avirulent serotypes show multiple bands, with strong banding between 23-40 kDa, as well as a protein of approximately 68 kDa.
Several other proteins associated with H. parasuis infections have been suggested. Two colonization proteins have been reported, designated P2 and P5, which are both immunogenic. P2, surprisingly, appeared to differ depending on serotype virulence. It is dominantly present as a 55 kDa protein in avirulent serotypes and 48 kDa in virulent serotypes. This protein shows homology to Haemophilus influenzae's P2 protein. Others have identified and described the upregulation of the TonB region of H. parasuis's genome. This region contains several genes that respond to iron-depleted environments. Specifically, a transferrin-binding protein was identified and shown to be upregulated when iron is restricted. As iron is sequestered in the host, it has been proposed that such genes may be important for pathogen survival within the host.
Additionally, a 42 kDa major outer membrane protein (MOMP) was detected using a polyclonal antibody directed against the 35 kDa MOMP of Pasturella multocida. Analysis of a potentially similar 42 kDa protein of Haemophilus ducreyi, a closely related species, characterized the protein as antigenically similar to OmpA. This class of heat-modifiable membrane protein was investigated further through the development of monoclonal antibodies against H. parasuis membrane preparations. Two monoclonal antibodies were used in this experiment, one against a 35 kDa OMP and a second against LPS. These monoclonal antibodies were reported to react specifically with the common serotypes and their potential value as diagnostic tools or potential vaccine targets was suggested.
Neuraminidase is another potential virulence factor. More than 90% of field isolates appear to produce neuraminidase. This enzyme is expressed late in the growth phase of H. parasuis and is correlated with both the exposure of necessary colonization receptors and the breakdown of mucin within the host.
H. parasuis can infect multiple sites of the host. As a result, clinical signs manifest differently based on the site of infection. The four primary forms of infection are Glässer's disease (fibrinous polyserositis), septicaemia (without polyserositis), myositis acuta (masseter muscle), and respiratory disease. Regardless of site of infection or infection type, symptoms of H. parasuis infection have been reported to be somewhat general. Increased temperature, apathy, and loss of appetite are commonly reported. Other common clinical symptoms have been reported to include cough, dyspnoea (shortness of breath), weight loss, lameness, lack of coordination, cyanosis, and exhaustion.
H. parasuis has become a major issue after specific-pathogen-free (SPF) herds became prevalent. In part due to the evolution of the hog production business, which includes the establishment of specific-pathogen free herds, H. parasuis has appeared as an economically significant pathogen. Typically, the infection targets naive animals, those housed with inadequate hygiene, or those fed poorly. Further, insecure transport and the commingling of different-aged pigs have contributed significantly to outbreaks. The combination of the higher concentrations of animals and the relative naivete of the swine population in these protected herds has been reported to have led to an increase in the incidence of H. parasuis induced disease. Complicating matters further is the fact that H. parasuis exists in several different regionally specific serotypes. It was reported that exposure or vaccination to one serotype did not necessarily protect against infection by others. As such, autogenous vaccine development was proposed as a control against unknown serotype spread. Due in part to such problems and the delay between autogenous bacterin generation and exposure to swine, a need arose for a cross-protective vaccine that could be administered with confidence regardless of regional serotype prevalence.
Treating H. parasuis infection with antibiotics has been proposed for immediate application upon the development of clinical signs. Unfortunately, the penetrative nature of the pathogen requires high doses of antibiotics to be effective and is often cost prohibitive.
Control via vaccination has been attempted with both commercial and autogenous vaccines. Diversity of H. parasuis serotypes has complicated vaccination regimens, as cross-protection is rare. Combined with the non-typeable strains, this plethora of antigenic profiles made vaccine development difficult.
Protection by vaccination against homologous challenge also has been proposed. A trio of studies suggested that a killed bacterin product could protect against homologous challenge when created with known serotypes and un-typed field isolates. The studies shed light on the use of autogenous vaccines to control outbreaks to reduce mortality rates.
Some had proposed using virulent strains to protect against heterologous challenge from other virulent strains. One study reported a bivalent vaccine containing serotypes 4 and 5 protected against serotypes 13 and 14. Others, however, failed to show cross-protection between serotypes 2 and 5.
Still others have proposed controlled exposure of piglets to low doses of live, virulent H. parasuis. However, due in part to damaging co-infections with other pathogens, such as porcine reproductive and respiratory syndrome virus (PRRSV), this approach has not been recommended as a functional control method.
As currently available methods of controlling various disease-causing infections are limited in effectiveness, in part due to the diversity of disease-causing agents such as H. parasuis, effective methods and compositions for treatment and prevention are needed, particularly a need to identify proteins that are cross-reactive that can permit the development of effective vaccines, in particular for treatment and prevention of infection byH. parasuis. 